Rotating machines such as steam turbines have profiled blades mounted on fixed or rotating parts of the machines that come into contact with a fluid flowing through the machine at very high speed when the machine is operating.
The blades of rotating machines have a leading edge facing towards the flow of the fluid impinging on the blades of the machine and a trailing edge towards the rear of the blade with reference to the direction of flow of the fluid.
In some cases, the blade of the rotating machine is heavily eroded by the fluid flowing in the vicinity of its leading edge.
In particular, in steam turbines, the blades of the final stage or stages of the turbine, i.e., the low-pressure stages, are subject to heavy erosion in the vicinity of their leading edge because the steam then has a high moisture content and conveys droplets of water that impact on the leading edge of the blade. The contact of the water droplets with the leading edge of the blade produces a phenomenon of erosion that causes loss of metal from the blade in a portion near its leading edge. After steam turbines have been operating for some time, a lack of balance of the rotor assembly can result from asymmetric wear of the blades of the rotor of the low-pressure stages of the turbines. This imbalance encourages vibrations leading to repetitive stresses on the rotor which can become dangerous and cause the rotor to break up.
If such vibrations are observed, it is essential to stop the turbine and therefore the installation including the turbine.
Very high operating losses can result from this stoppage in vast installations such as nuclear or thermal power stations.
The rotor blades that have been damaged must be repaired or replaced, and this can be a lengthy and costly operation.
To limit erosion of the leading edges of the blades of rotating machines, and in particular of turbine blades, it has been proposed to apply an anti-wear coating to a portion of the outer part of the blade near its leading edge and over a fraction of the length of the leading edge, referred to the radial direction of the rotating machine.
Such coating or surfacing, which must be effective at a high temperature on the blade when machined to its finished state, can degrade the mechanical properties of the metal constituting the blade or lead to deformation of the blade due to thermal stresses occurring during high temperature coating.
It has therefore been proposed, in particular in U.S. Pat. No. 5,351,395, to protect a turbine blade against wear in the vicinity of its leading edge by attaching in the area of the leading edge, and welding to the body of the blade, an insert made of a hard material that is resistant to wear or of a material that is hardened by heat treatment differently than the material constituting the body of the turbine blade. The blade reinforcing insert is fixed to the body of the blade, which can be a precision casting. The insert, which is attached and fixed to a part of the body of the blade designed for this purpose, has a crude shape and must be machined to the shape and to the profile of the turbine blade in the vicinity of its leading edge.
After welding the reinforcing insert to the cast blank of the turbine blade body, the molded blank of the blade body is finish machined and the insert is machined to shape. Finally, hardening heat treatment is carried out to develop the hardness and wear resistance characteristics of the insert.
However, the cast blank of the blade body has a shape close to the final shape of the blade and in particular a twisted and curved shape. The insert must have a profile assuring continuity with the body of the blade in its area near the leading edge. Because of the complex shape of the body of the blade it can be very difficult to design and to make the insert, which has a significant effect on the cost of bimetallic turbine blades made by the above method.